The importance to study and characterize semiconductors (bulk or thin film), semiconductor heterostructures (superlattices, quantum wells, heterojunctions) and semiconductor interfaces (Schottky barriers, metal-insulator-semiconductors, semiconductor-electrolyte, semiconductor-vacuum, etc.) assumes ever-greater significance, particularly as many of these semiconductors and semiconductor microstructures are fabricated by modern thin-film techniques such as molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), etc.
The materials and interfaces grown by MBE and MOCVD as well as other methods can be characterized by a variety of optical, electronic and structural methods including photoluminescence, photoluminescence excitation spectroscopy, absorption spectroscopy, modulation spectroscopy, Raman and resonant Raman scattering, cyclotron resonance, Hall effect, transmission electron microscopy, etc. Each of these tools provides specific information about the material of interest. For characterization purposes the experimental tools should be as simple and informative as possible. Many of the methods mentioned above are specialized and sometimes difficult to employ. For example, a number thereof, such as photoluminescence, photoluminescence excitation spectroscopy, absorption, cyclotron resonance, generally require cryogenic temperatures. Because of its simplicity and proven utility, photoreflectance has recently gained importance for the evaluation of semiconductor thin films and heterostructures.
As pointed out in the aforementioned applications, the basic idea of modulation spectroscopy is a very general principle of experimental physics. Instead of directly measuring an optical spectrum, the derivative with respect to some parameter is evaluated. This can easily be accomplished by external or internal modulation of some parameter of the sample or measuring system in a periodic fashion and measuring the corresponding normalized change in the optical properties. In photoreflectance, the built-in electric field of the materials is modulated by the photo-injection of electron-hole pairs created by a pump beam of wavelength .lambda..sub.p which is chopped at frequency .OMEGA..sub.m.
An improved apparatus utilizing photoreflectance, which will be described by reference to FIG. 1, is disclosed in the aforementioned application Ser. No. 07/408,903, which enables achievement of improved signal-to-noise ratios, to further eliminate problems encountered in the prior art apparatus and in particular to utilize novel computerized procedures to gain additional information on the characteristics of the materials examined.
Accordingly, it is an object of the present invention to provide an improved method for in-situ determination of the Fermi level in GaAs and similar materials by photoreflectance which avoids by simple means the shortcomings and drawbacks encountered in the prior art.
Another object of the present invention resides in a method which permits in-situ determination of the Fermi level in GaAs and similar materials with great accuracy by simple means.
A further object of the present invention resides in a method utilizing photoreflectance for determining the Fermi level of certain materials in situ, for example, in the MBE growth chamber of the material, which is simple to use, provides relatively great accuracy in the information which can be obtained and assures high reliability.
Still another object of the present invention resides in a method utilizing computer technologies to obtain in-situ information on the Fermi level of GaAs and similar materials.